Al-Cu-Mg sheet metals with low levels of residual stress

ABSTRACT

A metal sheet with a total thickness &gt;0.5 mm comprising an AlCuMg aluminum alloy consisting essentially of Al and, in percent by weight: - 3.5 &lt; Cu &lt; 5.0 - 1.0 &lt; Mg &lt; 2.0 - Si &lt; 0.25 - Fe &lt; 0.25 - Mn &lt; 0.55 -  all other elements: &lt;0.25 with 0&lt;Mn-2Fe&lt;0.2, optionally plated with another aluminum alloy with the thickness of the plating being no more than 12% of the total thickness of the sheet, the sheet having a recrystallization rate &gt;50% at all points and a deviation in recrystallization rate between surface and mid-thickness &lt;35%, the sheet having in the quenched and stretched state or in the quenched, stretched and annealed state, a deflection after machining to mid-thickness of a bar resting on two distant supports with a length l such that: fe&lt;0.14l2, where f is the deflection expressed in micrometers, e being the thickness of the sheet in mm and l is the length of the bar in mm.

FIELD OF THE INVENTION

The invention relates to heavy (>12 mm thick), average (between 3 and 12mm thick), or light (between 0.5 and 3 mm thick) sheet metals made froma high-strength Al--Cu--Mg aluminum alloy belonging to the 2000 series,in accordance with the designations of the Aluminum Association of theUnited States, which after quenching have a low level of residualstress, while retaining high static mechanical properties (tensilestrength, yield strength and elongation), excellent damage tolerance, alow crack propagation rate and good fatigue resistance, all of whichproperties are particularly well adapted to their use in aircraftconstruction. These sheets can be used uncoated or plated with anotheraluminum alloy having, for example, better corrosion resistance.

DESCRIPTION OF RELATED ART

The residual stress present in age hardened aluminum sheets results fromthe quenching which must be carried out in order to provide them withgood mechanical properties. The thermal shock caused by the rapidcooling from high temperatures required for the natural aging of thealloying elements produces extremely high internal stress.

This stress is troublesome because it causes substantial strain when thesheets are machined, which is frequently the case in aircraftconstruction. In order to reduce this stress, various processes forrelieving stress are used after quenching, for example a controlledstretching or compression which makes it possible to reduce the internalstress without affecting the properties of the product like a heattreatment would. The research in this area has essentially consisted ofoptimizing the stretching or compression operations required to ensureeffective stress relief.

In addition, much work has been done on the quenching operation itself.This operation is generally carried out by means of immersion in orspraying with cold water, and the cooling speeds obtained in this wayare often unnecessarily high. In effect, each alloy has a criticalquenching rate; if the cooling occurs more slowly than this criticalrate, it causes a decomposition of the solid solution which results in asubstantial reduction in the ultimate mechanical properties as well asthe damage tolerance. It must therefore be quenched faster than thiscritical rate, but it is useless to go much beyond it, since it is knownthat the more intense the cooling, the higher the internal stress.

Thus, a compromise must be found in order to optimize the quenching ofthe sheets with a cooling which prevents any reduction in the ultimatemechanical properties and minimizes the internal stress.

SUMMARY OF THE INVENTION

The object of the invention is to obtain, in sheet metals made from agehardening alloys of the Al--Cu--Mg type, a reduced level of residualstress after quenching, while maintaining static mechanical properties(tensile strength, yield strength and elongation) and a fatigueresistance which are as high as those in the current alloys, and whileimproving, in heavy sheets, the toughness in the various directions andthe crack velocity in the long-transverse (L-T) and transverse-long(T-L) directions, without changing anything in the procedures currentlyused for quenching and relieving stress.

The subject of the invention is a sheet metal with a thickness of >0.5mm made from an aluminum alloy with the following composition (% byweight):

    ______________________________________                                                    3.5 < Cu < 5.0                                                                1.0 < Mg < 2.0                                                                Si < 0.25                                                                     Fe < 0.25                                                                     Mn < 0.55                                                         ______________________________________                                    

all other elements <0.25

with 0<Mn-2Fe <0.2

possibly plated on 1 or 2 sides with another aluminum alloy having atotal thickness which does not exceed 12% of the total thickness of theplated sheet, which sheet has a recrystallization rate >50% at allpoints and a deviation between the recrystallization rate at the surfaceand that at mid-thickness <35%, and has in the quenched and stretchedstate, or the quenched, stretched and annealed state, a deflection fafter a machining to half-thickness of a bar resting on two distantsupports with a length l, such that:

    fe<0.14l.sup.2

preferably:

    fe<0.09l.sup.2

and even more preferably:

    fe<0.06l.sup.2

with f being measured in micrometers, e being the thickness of the sheetin mm and l also being measured in mm.

For light sheets with a thickness of <3 mm, the deflection is such that:

    fe<0.04l.sup.2

Preferably, the iron content is less than 0.2%, the silicon content lessthan 0.17% or even 0.10%, the copper content less than 4%, the magnesiumcontent less than 1.5%, and the manganese content less than 0.4%.

The sheets have a yield strength R₀.2 in the transverse-long directiongreater than 290 MPa in the quenched state, and greater than 400 MPa inthe annealed state. As for plated sheets such as, for example, thoseused in the manufacture of aircraft fuselages, they are generally platedon two sides with an aluminum alloy which is not very loaded and hasgood corrosion resistance, and each layer of plating can representbetween 4 and 6% of the total thickness in the lightest sheets, and upto 2 to 4% of the total thickness of sheets >1.6 mm thick, which meansthat the total thickness of the plating never exceeds 12% of this totalthickness. For these plated sheets, the yield strength in the L-T andT-L directions is greater than 270 and 380 MPa, respectively.

The sheets have a fatigue resistance, measured on flat test bars with astress concentration factor K_(t) =2.3 with a ratio R between theminimum and the maximum stress of 0.1, such that the stress acceptablefor a given number of cycles is greater than:

295 MPa for 10⁴ cycles

160 MPa for 10⁵ cycles

100 MPA for 10⁶ cycles

100 MPa for 10⁷ cycles

The heavy sheets >20 mm thick, made of an alloy with less than 0.2% Fehave a toughness in the quenched and stretched state, measured by thecritical stress intensity factor under plane strain K_(1c), in the L-Tand T-L directions which is respectively greater than 35 and 32 MPa√m,and preferably greater than 40 and 35 MPa√m.

In the quenched, stretched and annealed state, this toughness isrespectively greater than 28 and 25 MPa√m, and preferably greater than32 and 28 MPa√m.

The toughness measured in the S-L direction for sheets >35 mm thick isgreater than 22 and preferably 24 MPa√m in the quenched, stretchedstate, and greater than 18 and preferably 20 MPa√m in the quenched,stretched and annealed state. The heavy sheets also have, in the L-T andT-L directions, a crack velocity da/dn, for a loading with R=0.1, whichis less than: ##EQU1## The orientation code for the L-T, T-L and S-Ldirections is defined in the ASTM E 399 standard related tests for thetoughness of metallic materials.

The sheets with a thickness of less than 20 mm have a toughness measuredby the critical stress intensity factor under plane strain K_(cb), inthe T-L direction, greater than 110 MPa√m. It is measured on a test barwith a width of 405 mm, a notch length of 100 mm and a thickness equalto that of the sheet up to 6 mm and a thickness of 6 mm beyond that,which thickness is obtained after symmetrical surfacing.

DESCRIPTION OF THE INVENTION

In contrast with the research trends of the prior art, the inventorshave researched the reduction of residual stress at the level of themetallurgic parameters involved before quenching.

Since the possibilities for deviating from the compositions of theexisting alloys are limited for the major alloying elements (Cu and Mg)because of the high mechanical properties which must be obtained, theinventors sought modifications in the contents of the minor alloyingelements, and found that the best results in terms of reducing residualstress, and thus in terms of machining stability, were obtained when thecontents of iron and manganese by weight were such that:

    Mn<0.55% Fe<0.25% and 0<Mn-2Fe<0.2%

This indicates that the lower the iron content, the more the manganesecontent must be reduced. The iron content of Al--Cu alloys has atendency to be lowered regularly, for example as evidenced by theevolution, over the last 20 years, of the compositions registered withthe Aluminum Association for the alloys 2024, 2124, 2224 and 2324,whereas in these successive compositions, the Mn content has notchanged. Adjusting the outlet temperature of the hot rolling obtains,with the compositions according to the invention, a largelyrecrystallized microstructure with a recrystallization rate which isalways higher than 50% and a recrystallization gradient between thesurface and the core of the sheet which is always less than 35%. This isparticularly remarkable in heavy sheets which, at mid-thickness, have astructure which is substantially more recrystallized than the sheets ofthe prior art with the same composition in terms of major elements.

Contrary to what the metallurgist specializing in high-strength aluminumalloys might expect, this highly recrystallized structure and the low Mncontents, which participate in the age hardening of the alloy due to thefine precipitates of Al₂ OCu₂ Mn₃ and AlMn₆, do not affect the staticmechanical properties of the sheet in any significant way. Moreover, ithas been determined that the fatigue properties are also preserved,whereas a reduction in fatigue resistance might have been expected.

Furthermore, in the case of heavy sheets >20 mm thick, the inventorsunexpectedly determined that a largely recrystallized structure wouldlead to greater toughness in all directions, as measured by the criticalstress intensity factor under plane strain K_(1c) in accordance with theASTM E 399 standard.

Finally, these heavy sheets with a largely recrystallized structure havelower crack-velocities in the L-T and T-L directions than the sheets ofthe prior art with the same composition in terms of major elements.Thus, they make it possible to obtain an advantageous compromise betweenthe static mechanical properties and the damage tolerance properties(toughness and crack velocity).

For light sheets, the inventors determined that the compositionaccording to the invention had a positive influence on the elongation inthe transverse-long direction of the sheet, in contrast to the generallyaccepted idea that high Mn and Fe contents have a favorable effect onthis elongation, since the fine precipitates from the manganese make itpossible to homogenize the strain by limiting the formation of bands ofstrain. Likewise, it was generally accepted by metallurgists that forlight or average sheets, an extremely recrystallized and fine-grainedstructure, which was recognized to be beneficial to elongation, waspreferably obtained with high Mn and Fe contents.

Thus the reduction in the Mn-2Fe content below the 0.2% threshold, inlight sheets as well as heavy sheets, leads not only to a reduction inresidual stress which results in better machining stability, but also toa set of usage properties which are particularly advantageous foraircraft construction. It is not desirable, however, for the value ofMn-2Fe to become negative, since in that case a degradation of themechanical properties is observed without any additional gain in thereduction of internal stress.

The sheets according to the invention have, in the quenched andstretched state or in the quenched, stretched and annealed state, alevel of residual stress such that the deflection f measured after amachining-to half-thickness of a bar resting on two distant supportswith a length 1 is such that:

    fe<0.14l.sup.2

with f being measured in micrometers, and the thickness e of the sheetand the length l being expressed in mm.

This deflection is measured in the following way. Two bars are takenfrom the sheet with the thickness e: one called the direction L bar,which has a length b in the direction of the length of the sheet(direction L), a width of 25 mm in the direction of the width of thesheet (direction TL) and a thickness e which corresponds to the fullthickness of the sheet (direction TC); the other, called the directionTL bar, has 25 mm in the direction L, b in the direction TL and e in thedirection TC.

Each bar is machined to half-thickness and the deflection is measured atthe mid-length of the bar. This deflection represents the level ofinternal stress of the sheet and its ability not to be deformed-duringmachining.

For heavy sheets with a thickness greater than 20 mm, the length b ofthe bars is 5e+20 mm. The machining is a progressive mechanicalmachining in 2 mm passes. The deflection at mid-length is measured tothe nearest micrometer with the aid of a comparator, at the center ofthe bar, which is positioned between two distant knives with l=5e, whichbar extends 10 mm beyond both sides of the knives.

For sheets with a thickness of <20 mm, the length b of the bar is 400 mmand the length l used to measure the deflection is set at 300 mm.

For thicknesses between 8 and 20 mm, the machining is a mechanicalmachining in 1 mm passes. Below 8 mm, the machining is a chemical one ina soda bath. One side of the bar is protected by means of a flexibleplastic mask put in place before the test. The sample is removed fromthe etching bath and its thickness is checked every 15 minutes.

For light sheets with a thickness of <2 mm, the method is slightlydifferent. The measurement of the deflection is carried out with the barplaced on its side (length, half-thickness) on a sheet of paper markedin millimeters, which is itself placed on a horizontal surface, makingit possible to measure the deflection to the nearest 0.5 mm, byeliminating the influence of the dead weight of the bar and the force ofthe comparator on the deflection at mid-length.

The inventors also determined that the isotropy of strain could beimproved. Thus, in the sheets according to the invention, the deflectionmeasured on the bars in the long direction and in the directiontransverse to the rolling were such that:

    (direction L deflection)<1.5(direction TL deflection)

For light and average sheets <12 mm thick, it was determined that theroughness after chemical machining was less than 6 micrometers, and forsheets less than <4 mm thick, less than 3 micrometers.

The invention also applies to aluminum alloy products other than sheetmetals, for example extruded, forged, or die-formed products. In thiscase, the thickness e of the bar is the local thickness of the piece,and if this thickness is not constant, a surfacing can be carried out inorder to produce a bar of constant thickness for the measurement of thedeflection.

These products have a yield strength >290 MPa in the quenched andde-stressed state, and >400 MPa in the quenched, de-stressed andannealed state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 10 represent the comparative results mentioned in thethree examples so as to show the improvements in the properties obtainedby the sheets according to the invention.

FIGS. 1 and 2 show the improvement in machining stability in the longdirection (L) and in the traverse-long direction (TL) for heavy sheets.

FIG. 3 shows the improvement in the isotropy of the machining stabilityfor these sheets in the directions L and TL.

FIGS. 4, 5 and 6 show, for these same sheets, the improvement intoughness in the directions L-T, T-L, and S-L.

FIG. 7 illustrates the results in terms of fatigue resistance.

FIG. 8 shows the relative improvement in crack velocity.

FIG. 9 shows the improvement in elongation in the direction T-L in lightsheets.

FIG. 10 shows the improvement in machining stability for light sheets.

FIGS. 11 and 12 illustrate the results related to machining stabilityand crack velocity for average sheets.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The inventors carried out several semi-continuous castings of platedsheets made from different alloys of the 2024 type, in the nomenclatureof the Aluminum Association. All of the plates had the same dimensionsand were cast using the same procedures. They were subjected to astandard transformation sequence for heavy sheets, that is: reheatingafter homogenization, hot rolling, natural aging, quenching with coldwater by spraying, controlled stretching in accordance with the EN 515standard between 1.5 and 3%, age hardening at an ambient temperature. Inthis way, sheets were obtained which had a thickness of 55 mm in theT351 state, in the nomenclature of the Aluminum Association. Thecompositions of the cast alloys were the following:

    ______________________________________                                        Alloy                                                                              Si     Fe     Cu   Mn    Mg   Cr   Zn   Ti   Zr                          ______________________________________                                        A1   0.11   0.23   4.32 0.63  1.43 0.022                                                                              0.11 0.02 0.014                       A2   0.08   0.17   4.52 0.52  1.40 0.008                                                                              0.10 0.02 0.002                       A3   0.08   0.16   4.48 0.51  1.41 0.007                                                                              0.08 0.02 0.002                       A4   0.08   0.15   4.32 0.37  1.29 0.005                                                                              0.05 0.02 0.001                       A5   0.08   0.16   4.44 0.54  1.30 0.008                                                                              0.08 0.02 0.002                       ______________________________________                                    

The following measurements were carried out on these sheets:

deflection after machining according to the method described above. Itwas noted that the deflection obtained in the sheets made from thealloys A2, A3 and A4 according to the invention is lower, particularlyin the direction L, than for those made from the alloys A1 and A5outside the invention.

static mechanical properties (tensile strength R_(m), 0.2% yieldstrength R₀.2, elongation at rupture A) in the directions TL (transverseto the rolling) and TC (transverse-short).

toughness measured in the directions L-T, T-L and S-L in accordance withthe ASTM E399 and B645 standards. The improvement appears in FIGS. 4through 6.

recrystallization rate at the surface, at quarter thickness and atmid-thickness, measured from micrographs. All of the above results arearranged in Table 1.

fatigue life measured in the directions L and T-L according to the ASTME466 standard, for sample No. 3 (the alloy A1 outside the invention) andsample No. 9 (the alloy A4 according to the invention). The test barsare 3 mm flat test bars taken from the sheets at quarter thickness. Themachining of a central hole makes it possible to have a stressconcentration factor K_(t) =2.3. The loading is with a ratio R ofminimum stress to maximum stress of 0.1. The results, indicated in Table2, are roughly identical in the directions L and T-L. They are shown inFIG. 7, and quite similar results are noted for the two alloys.

crack velocity da/dn, also for samples No. 3 and 9, measured in thedirections T-L and L-T in accordance with the ASTM E647 standard, with aratio R=0.1, for values of ΔK between 10 and 25 MPa√m. The test bars areCT 35 test bars taken from the sheets at quarter thickness. The results,indicated in Table 3, are quite similar in both directions. It is notedin FIG. 8 that the crack velocities are lower in sample No. 9 than insample No. 3.

EXAMPLE 2

Plates made from an alloy of the 2024 type were cast semi-continuously,then subjected to a standard transformation sequence for light platedsheets, namely: reheating, simultaneous hot-rolling with two platingsheets made from the alloy 1070, cooling, cold rolling, natural aging,quenching with cold water, finishing by pass rolling and controlledstretching, age hardening at an ambient temperature. In this way, sheetswith a thickness of 1.6 mm in the T351 state were obtained which had, oneach side, a plating thickness representing 5% of the thickness of thesheet.

The compositions of the alloy 2024 were the following:

    ______________________________________                                        Alloy                                                                              Si     Fe     Cu   Mn    Mg   Cr   Zn   Ti   Zr                          ______________________________________                                        A6   0.09   0.19   4.38 0.63  1.50 0.013                                                                              0.10 0.024                                                                              0.014                       A7   0.079  0.17   4.36 0.52  1.30 0.012                                                                              0.013                                                                              0.022                            ______________________________________                                    

The following properties were measured in these sheets:

the deflection after machining according to the method described above.It is noted that in the alloy A7 according to the invention, thisdeflection was clearly reduced, both in the direction L and in thedirection TL, relative to the alloy A6 outside the invention, and thatit verifies the relation: fe<0.04 l².

the static mechanical properties in the direction TL (an average of 2test bars taken in the direction transverse to the rolling and anaverage of 4 sheets per alloy).

The results are summarized in Table 4. An improvement in the elongationin the direction TL of the alloy A7 relative to A6 is noted in FIG. 9,and a reduction in the deflection during machining of A7 relative to A6is noted in FIG. 10.

EXAMPLE 3

Plates with the same dimensions were cast semi-continuously, using thesame casting procedure. These plates were subjected to a standardtransformation sequence for average sheets, that is: reheating, hotrolling, natural aging, quenching with cold water, controlledstretching, age hardening at an ambient temperature. In this way, sheetswith a thickness of 12 mm in the T351 state were obtained, which had thefollowing composition:

    ______________________________________                                        Alloy  Si         Fe     Cu      Mn   Mg                                      ______________________________________                                        A8     0.08       0.17   4.45    0.53 1.46                                    ______________________________________                                        Alloy  Cr         Ni     Zn      Ti   Zr                                      ______________________________________                                        A8     0.007      0.005  0.06    0.02 0.002                                   ______________________________________                                    

The following properties were measured in these sheets:

the deflection after machining according to the procedure describedabove,

the static mechanical properties in the direction TL (transverse to therolling),

the recrystallization rate on the surface, at quarter thickness and atmid-thickness.

The results are presented in Table 5 and illustrated in FIGS. 11 and 12.

                                      TABLE 1                                     __________________________________________________________________________    SHEET    DEFLECTION                                                                            RECRYSTALLIZATION  MECHANICAL PROPERTIES                        e.    (micrometers)       Surface/Mid-th.                                                                      TL       TC       TOUGHNESS               No.                                                                              (mm)                                                                             Alloy                                                                            fL  fTL Surface                                                                           Quarter                                                                           Mid-th.                                                                           Deviation                                                                            Rm R0.2                                                                             A %                                                                              Rm R0.2                                                                             A %                                                                              L-T                                                                              T-L                                                                              S-L               __________________________________________________________________________    1  55 A1 210 120                    478                                                                              351                                                                              13.6                                                                             431                                                                              306                                                                              5.9                        2  55 "  231 55                     468                                                                              342                                                                              15.7                                                                             432                                                                              306                                                                              6.1                        3  55 "  207 79  79  58  30  49     470                                                                              341                                                                              15.3                                                                             432                                                                              306                                                                              6.2                                                                              37.1                                                                             32.2                                                                             22.2              4  55 A2  57 43  99  95  71  28     461                                                                              330                                                                              16.5                                                                             419                                                                              315                                                                              7  44.4                                                                             38.1                 5  55 "   46 33  100 95  69  31     462                                                                              329                                                                              17.2                                                                             417                                                                              314                                                                              6.3                                                                              44 40.9                 6  55 "   42 31  100 96  68  32     462                                                                              329                                                                              16.9                                                                             422                                                                              312                                                                              7.1                                                                              45.3                                                                             38.5                 7  55 A3  57 62  97  84  63  34     468                                                                              343                                                                              17.7                                                                             421                                                                              320                                                                              5.4                        8  55 "  100 70  96  68  62  34     481                                                                              358                                                                              14.7                                                                             422                                                                              315                                                                              4.9                                                                              43.2                                                                             37.2                                                                             26.1              9  55 A4  49 73  99  93  70  29     463                                                                              332                                                                              14.4                                                                             425                                                                              299                                                                              8.5                                                                              51.8                                                                             43.8                                                                             29.6              10 55 A5 156  9  95  82  64  31     470                                                                              344                                                                              16.5                                                                             425                                                                              312                                                                              6.1                        11 55 "  128  1  96  86  62  34     468                                                                              336                                                                              17.4                                                                             418                                                                              314                                                                              5.1                        12 55 "  150 25  99  88  70  29     469                                                                              338                                                                              16.1                                                                             418                                                                              314                                                                              6                          __________________________________________________________________________     Rm, R0.2 in MPa                                                               Toughness in MPa √m                                               

                  TABLE 2                                                         ______________________________________                                        Fatigue results in sheet Nos. 3 and 9                                         (Kt = 2.3, R = 0.1)                                                           Sheet                   Sheet                                                 No. 3 Max.    Endurance No. 9                                                 Direc-                                                                              stress  (No. of   Direc-                                                                              Max. stress                                                                           Endurance                               tion  (MPa)   cycles)   tion  (MPa)   (No. of cycles)                         ______________________________________                                        TL    260      21000    TL    300     10700                                   L     260      20000    L     300     15400                                   TL    230      31000    TL    280     23200                                   L     230      33000    L     280     22500                                   TL    230      33000    TL    260     25600                                   L     230      35000    L     260     22600                                   TL    230      35000    TL    240     30200                                   TL    210      47000    L     240     33000                                   L     210      51000    TL    222     58800                                   TL    180     131000    L     210     60800                                   L     180     140000    TL    200     95100                                   TL    160     279000    TL    190     101600                                  L     160     150000    L     190     110000                                  TL    150     15553000  TL    180     182800                                  TL    170     147000    L     180     190000                                  L     170     173000    TL    160     332000                                  TL    160     420000    L.    160     700000                                  L     160     256000    TL    150     589700                                  TL    170     121000    L     150     434000                                  L     170     139000    TL    140     9567000                                 TL    160     234000    L     140     7834500                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Sheet No. 3           Sheet No. 9                                             Delta K   da/dn       Delta K   da/dn                                         (MPa√m)                                                                          (mm/cycle)  (MPa√m)                                                                          (mm/cycle)                                    ______________________________________                                        10        1.0E-04     10        2.0E-05                                       15        4.0E-04     15        1.0E-04                                       20        7.0E-04     20        6.0E-04                                       25        2.5E-03     25        2.0E-03                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                          MECHANICAL                                                                    PROPERTIES                                                  SHEET       DEFLECTION  TL                                                    Sheet               (micrometers)                                                                           Rm                                              No.  Alloy  ei (mm) fL    fTL   (MPa) R0.2 (MPa)                                                                            A %                             ______________________________________                                        13   A6     1.6     4000  3000  440   305     20.05                           14   A6     1.6     3000  4000  440.5 301.5   20.95                           15   A6     1.6     4000  3500  441   298.5   21.55                           16   A6     1.6     3500  3000  443   301     21.25                                       Average 3625  3375  441.1 301.5   21.0                            17   A7     1.6      500    0   439.5 294     24.55                           18   A7     1.6     1500  1500  438.5 277.5   24.4                            19   A7     l.6     2000  1500  440   290     23.85                           20   A7     1.6     1000    0   441   289.5   25                                          Average 1250   750  439.8 287.8   24.5                            ______________________________________                                    

                                      TABLE 5                                     __________________________________________________________________________                DEFLECTION                    MECHANICAL PROPERTIES:              SHEET       (micrometers)                                                                         RECRYSTALLIZATION     TL                                  Sheet No.                                                                          e. (mm)                                                                           Alloy                                                                            fL  fTL Surface                                                                           Quarter                                                                           Core                                                                             Surface-Core Deviation                                                                   Rm (MPa)                                                                           R0.2 (MPa)                                                                          A %                      __________________________________________________________________________    21   12  A8 240 480  90 90  67 23         465  335   15                       22   12  A8 710  90 100 99  97  3         470  339   15                       __________________________________________________________________________

What is claimed is:
 1. A metal sheet with a total thickness >0.5 mm,comprising an AlCuMg aluminum alloy consisting essentially of Al and, in% by weight:

    ______________________________________                                                    3.5 < Cu < 5.0                                                                1.0 < Mg < 2.0                                                                Si < 0.25                                                                     Fe < 0.25                                                                     Mn < 0.55                                                         ______________________________________                                    

all other elements: <0.25 with 0<Mn-2Fe<0.2,optionally plated withanother aluminum alloy with the thickness of the plating being no morethan 12% of the total thickness of the sheet, said sheet having arecrystallization rate >50% at all points and a deviation inrecrystallization rate between surface and mid-thickness <35%, saidsheet having in the quenched and stretched state or the quenched,stretched and annealed state, a deflection after a machining tomid-thickness of a bar resting on two distant supports with a length lsuch that:

    fe<0.14l.sup.2,

with f being the deflection expressed in micrometers, e being thethickness of the sheet in mm and l being the length of the bar in mm. 2.The sheet according to claim 1, wherein

    fe<0.09l.sup.2.


3. The sheet according to claim 2, wherein

    fe<0.06l.sup.2.


4. A sheet according to claim 3 with a total thickness between 0.5 and 3mm and

    fe<0.04l.sup.2

which is plated and has in the quenched, stretched and annealed state ayield strength >380 MPa.
 5. A sheet according to claim 1 in whichFe<0.20.
 6. A sheet according to claim 1 in which Si<0.17.
 7. The sheetaccording to claim 6 in which Si<0.10.
 8. A sheet according to claim 1in which Cu<4.0.
 9. A sheet according to claim 1 in which Mg<1.5.
 10. Asheet according to claim 1, wherein Mn<0.4.
 11. A sheet according toclaim 1 having, between the bars machined to half-thickness in thedirections L and TL, an isotropy of deflection after machining such that(bar deflection L)<1.5×(bar deflection TL).
 12. A sheet according toclaim 1, wherein it has in the quenched and stretched state a yieldstrength in the direction TL>290 MPa.
 13. A sheet according to claim 1having in the quenched, stretched and annealed state a yield strength inthe direction TL>400 MPa.
 14. A plated sheet according to claim 1 havingin the quenched and stretched state a yield strength in the directionTL>270 MPa.
 15. A sheet according to claim 1 which is plated and has inthe quenched, stretched and annealed state a yield strength >380 MPa.16. A sheet according to claim 1 having a fatigue resistance such thatthe maximum stress acceptable at a given number of cycles isrespectively greater than:MPa for 10⁴ cycles MPa for 10⁵ cycles MPA for10⁶ cycles MPa for 10⁷ cycles.
 17. A sheet according to claim 1 of totalthickness >20 mm and having in the quenched, stretched state a toughnessK_(1c) in the direction L-T >35 MPa√m.
 18. A sheet according to claim 1of total thickness >20 mm having in the quenched, stretched state atoughness K_(1c) in the direction T-L >32 MPa√m.
 19. The sheet accordingto claim 17 having in the quenched, stretched state a toughness K_(1c)in the direction L-T >40 MPa√m.
 20. The sheet according to claim 18having in the quenched, stretched state a toughness K_(1c) in thedirection T-L >35 MPa√m.
 21. A sheet according to claim 1 of totalthickness >35 mm and having a toughness K_(1c) in the direction S-L >22MPa√m.
 22. The sheet according to claim 21 having a toughness in S-L >24MPa√m.
 23. A sheet according to claim 1 of total thickness >20 mm andhaving in the quenched, stretched and annealed state a toughness K_(1c)in the direction L-T greater than 28 MPa√m.
 24. A sheet according toclaim 1 of total thickness >20 mm and having in the quenched, stretchedand annealed state a toughness K_(1c) in the direction T-L >25 MPa√m.25. The sheet according to claim 23 having a toughness K_(1c) in thedirection L-T >32 MPa√m.
 26. The sheet according to claim 24 having atoughness K_(1c) in the direction T-L >28 MPa√m.
 27. A sheet accordingto claim 23 of total thickness >35 mm and having in the quenched,stretched and annealed state a toughness K_(1c) in the direction S-L >18MPa√m.
 28. The sheet according to claim 27 having in the quenched,stretched and annealed state a toughness in the direction S-L >20 MPa√m.29. A sheet with a thickness >20 mm according to claim 1 having a crackvelocity da/dn less than: ##EQU2##30.
 30. A sheet according to claim 1of total thickness <20 mm and having a toughness K_(cb) in thedirection. T-L >110 MPa√m.
 31. A sheet according to claim 1 with athickness <12 mm and having a roughness after chemical machining <6micrometers.
 32. A sheet with a thickness <4 mm according to claim 31having a roughness after chemical machining <3 micrometers.
 33. Anextruded, forged, or die-formed product comprising an AlCuMg alloyconsisting essentially of Al and, in % by weight:

    ______________________________________                                                    3.5 < Cu < 5.0                                                                1.0 < Mg < 2.0                                                                Si < 0.25                                                                     Fe < 0.25                                                                     Mn < 0.55                                                         ______________________________________                                         all other elements <0.25

with: 0<Mn-2Fe<0.2having a recrystallization rate >50% at all points anda deviation in recrystallization rate between surface and mid-thickness<35%, and having in the quenched state or the quenched and annealedstate a deflection f after a machining to half-thickness of a barresting on two distant supports with a length l, such that:

    fe<0.14l.sup.2

with f being measured in micrometers, e being the average localthickness of the product at the measurement point and l also beingmeasured in mm.
 34. The product according to claim 33 having in aquenched, de-stressed state a yield strength R₀.2 >290 MPa.
 35. Theproduct according to claim 33 having in a quenched, de-stressed andannealed state a yield strength >400 MPa.
 36. The product according toclaim 33, wherein fe<0.09l².